Preface

Assembly language programming develops a very basic and low level understanding of the computer. In higher level languages there is a distance between the computer and the programmer. This is because higher level languages are designed to be closer and friendlier to the programmer, thereby creating distance with the machine. This distance is covered by translators called compilers and interpreters. The aim of programming in assembly language is to bypass these intermediates and talk directly with the computer.

There is a general impression that assembly language programming is a difficult chore and not everyone is capable enough to understand it. The reality is in contrast, as assembly language is a very simple subject. The wrong impression is created because it is very difficult to realize that the real computer can be so simple. Assembly language programming gives a freehand exposure to the computer and lets the programmer talk with it in its language. The only translator that remains between the programmer and the computer is there to symbolize the computer’s numeric world for the ease of remembering.

To cover the practical aspects of assembly language programming, IBM PC based on Intel architecture will be used as an example. However this course will not be tied to a particular architecture as it is often done. In our view such an approach does not create versatile assembly language programmers. The concepts of assembly language that are common across all platforms will be developed in such a manner as to emphasize the basic low level understanding of the computer instead of the peculiarities of one particular architecture. Emphasis will be more on assembly language and less on the IBM PC.

Before attempting this course you should know basic digital logic operations of AND, OR, NOT etc. You should know binary numbers and their arithmetic. Apart from these basic concepts there is nothing much you need to know before this course. In fact if you are not an expert, you will learn assembly language quickly, as non-experts see things with simplicity and the basic beauty of assembly language is that it is exceptionally simple. Do not ever try to find a complication, as one will not be there. In assembly language what is written in the program is all that is there, no less and no more.

After successful completion of this course, you will be able to explain all the basic operations of the computer and in essence understand the psychology of the computer. Having seen the computer from so close, you will understand its limitations and its capabilities. Your logic will become fine grained and this is one of the basic objectives of teaching assembly language programming.

Then there is the question that why should we learn assembly language when there are higher leve l languages one better than the other; C, C++, Java, to name just a few, with a neat programming environment and a simple way to write programs. Then why do we need such a freehand with the computer that may be dangerous at times? The answer to this lies in a very simple example. Consider a translator translating from English to Japanese. The problem faced by the translator is that every language has its own vocabulary and grammer. He may need to translate a word into a sentence and destroy the beauty of the topic. And given that we do not know


Japanese, so we cannot verify that our intent was correctly conveyed or not. Compiler is such a translator, just a lot dumber, and having a scarce number of words in its target language, it is bound to produce a lot of garbage and unnecessary stuff as a result of its ignorance of our program logic. In normal programs such garbage is acceptable and the ease of programming overrides the loss in efficiency but there are a few situations where this loss is unbearable.

Think about a four color picture scanned at 300 dots per inch making 90000 pixels per square inch. Now a processing on this picture requires 360000 operations per square inch, one operation for each color of each pixel. A few extra instructions placed by the translator can cost hours of extra time. The only way to optimize this is to do it directly in assembly language. But this doesn’t mean that the whole application has to be written in assembly language, which is almost never the case. It’s only the performance critical part that is coded in assembly language to gain the few extra cycles that matter at that point.
Consider an arch just like the ones in mosques. It cannot be made of big stones alone as that would make the arch wildly jagged, not like the fine arch we are used to see. The fine grains of cement are used to smooth it to the desired level of perfection. This operation of smoothing is optimization. The core structure is built in a higher level language with the big blocks it provides and the corners that need optimization are smoothed with the fine grain of assembly language which allows extreme control.

Another use of assembly language is in a class of time critical systems called real time systems. Real time systems have time bound responses, with an upper limit of time on certain operations. For such precise timing requirement, we must keep the instructions in our total control. In higher level languages we cannot even tell how many computer instructions were actually used, but in assembly language we can have precise control over them. Any reasonable sized application or a serious development effort has nooks and corners where assembly language is needed. And at these corners if there is no assembly language, there can be no optimization and when there is no optimization, there is no beauty. Sometimes a useful application becomes useless just because of the carelessness of not working on these jagged corners.

The third major reason for learning assembly language and a major objective for teaching it is to produce fine grained logic in programmers. Just like big blocks cannot produce an arch, the big thick grained logic learnt in a higher level language cannot produce the beauty and fineness assembly language can deliver. Each and every grain of assembly language has a meaning, nothing is presumed. You have to put together these grains, the minimum number of them to produce the desired outcome. Just like a “for” loop in a higher level language is a block construct and has a hundred things hidden in it, but using the grains of assembly language we do a similar operation with a number of grains but in the process understand the minute logic hidden beside that simple “for” construct.

Assembly language cannot be learnt by reading a book or by attending a course. It is a language that must be tasted and enjoyed. There is no other way to learn it. You will need to try every example, observe and verify the things you are told about it, and experiment a lot with the computer. Only then you will know and become able to appreciate how powerful, versatile, and simple this language is; the three properties that are hardly ever present together.

Whether you program in C/C++ or Java, or in any programming paradigm be it object oriented or declarative, everything has to boil down to the bits and bytes of assembly language before the computer can even understand it.